1. Field of the Invention
The invention relates to a method of clock signal recovery and synchronization for the reception of information elements transmitted in bursts by an ATM network. It also relates to a device for the implementation of the method.
It may be recalled, first of all, that an ATM (Asynchronous Transfer Mode) network enables the transmission of digital elements having various bit rates that could be high: indeed these bit rates could go up to 622 megabits per second and more.
Transmission in ATM mode is done by the transfer of information elements of constant length. The elementary quantity of information elements transmitted is equal to 48 bytes. The information block transmitted in ATM is called a "cell" and is identified by a five-byte label called a header. The total length of a cell is therefore equal to 53 bytes.
The invention can be applied especially to the recovery of clock signals and to synchronization for the reception of cells such as these transmitted in bursts on a network.
2. Description of the Prior Art
The reception of ATM cells transmitted in burst mode raises particular problems of clock signal recovery and synchronization. Indeed, in this burst mode of transmission, the cells are phase-shifted with respect to one another and are separated by periods during which the transmission channel is at rest. The rephasing and the resynchronization should therefore be done systematically at each new cell. To carry out these two operations, a method has been devised for preceding the cell proper by a sequence of bits that are called a precursor. This precursor has two parts processed independently by presently used reception systems in such a way as to obtain these two clock signal recovery and resynchronization operations. Of the two parts forming the precursor, one is called the preamble and the other is called the delimiter.
The preamble is used solely for the rephasing of the incoming cell. To this end, the receiver system receives a train of data elements that is a random binary train encoded by an NRZ type encoding controlled at transmission by a clock signal HE having the frequency F. This clock signal is not transmitted with the data elements. However, at reception, there is available a local clock signal HL having the frequency F' equal to F or very slightly different from F, without any phase relationship with the clock signal HE. It is therefore necessary, at reception of the data train, to delay these data elements at input so as to rephase them properly with the local clock signal HL. The delay to be introduced is not the same for all the data elements for they generally come from transmitters located at different distances. The delay should therefore be recomputed at the start of each cell. To make it possible to carry out this operation swiftly and efficiently, the preamble comprises numerous edges and generally has the following form: 101010 . . . 010101.
However, the rephasing system does not have instantaneous effect, and the beginning of the preamble often shows reception errors, bits having been omitted or even duplicated at output of the rephasing system. Given the extremely repetitive nature of the preamble, it is impossible to know how many bits have been lost and hence that bit of the preamble from which the reception actually begins.
The preamble makes it possible to carry out the "bit" rephasing but not the "byte" rephasing (or resynchronization).
The preamble is even more accentuated when there are transmission errors in the preamble.
The delimiter, for its part, is designed to carry out the operation of resynchronization. It is a known binary word that is inserted between the preamble and the cell proper, and the recognition of this delimiter makes it possible to have knowledge, unambiguously, of that bit of the data train at which the first byte of useful information begins. Unfortunately, should there be any error during the transmission of the delimiter, it will not be possible to carry out the byte synchronization and the cell will be lost.
In the even of error in the transmission line, the difficulties will be further increased by the fact that the size of the precursor must be as small as possible so as not to cause deterioration in the useful bit rate of the channel.
Thus, so as not to cause any deterioration in the useful bit rate of the channel, it is necessary for the precursor to be short. The rephasing should consequently be done very speedily. This rules out resorting to analog techniques that are used in a standard way in DC mode (filtering, PLL) for which preambles of more than about ten bits are necessary.
The possible presence of transmission errors among the bits used for the rephasing and for the resynchronization therefore entails the risk of a loss of cells, this risk being all the greater as this information is less redundant because of its brevity.
Presently existing systems, used to carry out these rephasing and resynchronization operations do not take account, in practice, of the possible presence of transmission errors. We may, however, cite the publication (ISH 91) by A. ISHIKURA et al. of the firm NTT entitled "A cell-based multipoint ATM transmission system for a passive double-start access network", IEEE Workshop on Local Optical Networks, September 91, Tokyo Art. G.3.3., on a system that enables the correction of a single error (i.e. one error bit) in the delimiter. However, the approach provided by this system cannot be used to resolve all the problems encountered by the Applicant.